1. Field of the Invention
The present invention relates to a composite ferrite composition exhibiting excellent high frequency property and further relates to an electronic device employing the same.
2. Description of the Related Art
Recently, a higher frequency band is used for mobile phones, PC and the like, and several GHz standards already exist. In line with that, noise reduction products applicable to the above high frequency signals are demanded. As a representative example, a multilayer chip coil is exemplified.
Electric properties of the multilayer chip coil can be evaluated by analyzing impedance data. The impedance characteristics are significantly influenced by permeability of element body materials and also its frequency properties up to 100 MHz. Further, the impedance in GHz band is influenced by stray capacitance between facing electrodes of the multilayer chip coil. As a method for reducing the stray capacitance between the facing electrodes of the multilayer chip coil, extension of a distance between the facing electrodes, area reduction of the facing electrodes, and decrease of dielectric constant between the facing electrodes are exemplified.
For Patent Document 1 described below, in order to reduce the stray capacitance, terminals are formed at both ends in a direction of magnetic flux, which is generated due to energization of a coil. In the invention of Patent Document 1, it enables to extend a distance between an internal electrode and a terminal electrode and also enables a reduction of the facing area between the internal electrode and the terminal electrode. With this, it is expected that the frequency properties can be improved up to higher frequencies.
However, in the invention of Patent Document 1, the stray capacitance between internal electrodes is not reduced and there is room for further improvement for this matter. Further, it is an improvement method to change the structure by extending a distance between internal electrodes and also reducing the area of the internal electrodes, and this brings about a significant influence on other properties, size and shape. The extension of the distance between internal electrodes influences the size of products and therefore it is difficult to apply the invention of Patent Document 1 to chip parts that miniaturization is demanded. Further, for the area reduction of internal electrodes, there is a problem that the DC resistance increases.
Currently, as element body materials for a multilayer chip coil, it is often the case that Ni—Cu—Zn based ferrite is used. In order for co-firing with Ag which is used as an internal electrode, Ni—Cu—Zn based ferrite is selected since it is magnetic ceramics capable of firing at 900° C. temperature. The dielectric constant of Ni—Cu—Zn based ferrite is approximately 14 to 15, and it is said that there is room for further reduction. However, it is difficult to reduce the dielectric constant of Ni—Cu—Zn based ferrite and some sort of improvement approach is necessary.
Further, for Patent Document 2 described below, Ni—Cu—Zn based ferrite and low dielectric constant non-magnetic material are mixed and composite materials thereby obtained are used as element body materials. As for the low dielectric constant non-magnetic materials, silica glass, borosilicate glass, steatite, alumina, forsterite, and zircon are exemplified.
In the invention of Patent Document 2, by mixing ferrite with low dielectric constant non-magnetic material, the dielectric constant is reduced. Further, in the invention of Patent Document 3, the application of foamed ferrite is indicated. Specifically, in Patent Document 3, burned-out materials are mixed in the magnetic ceramics and fired to form holes, and resin or glass is impregnated therein. By providing holes, the low dielectric constant can be achieved. Further, by impregnating the hole with resin or glass, it enables to cover the demerit of the foamed ferrite that the strength gets weaken.
However, for Patent Document 2, when glass-based materials are determined as a main component, the decrease of the permeability μ becomes prominent. It is considered that this is caused by an inhabitation of grain growth of magnetic materials and a division of magnetic path. Further, a reaction between ferrite and glass remarkably appears. With this, a heterophase is formed and thereby insulation resistance deteriorates. Therefore, for the glass-based materials, when co-firing with Ag-based conductor, there is a high possibility of short circuit, so that it is not suitable as a multilayer coil employing Ag-based conductor.
On the other hand, for ceramic materials such as steatite, alumina, forsterite, and zircon, it is considered that deterioration of insulation resistance is small. However, there is a problem with sinterability and it is considered difficult to sinter composite materials at a firing temperature 900° C., which is capable of co-firing with internal electrodes Ag.
Further, for the invention of Patent Document 3, there are no problems with properties and sinterability. However, a number of holes are included in ferrite and terminal electrodes cannot directly be attached. Therefore, for example, it is necessary to use ferrite having less holes on the portion where terminal electrodes are formed, so that the structure becomes complicated. Further, a particle diameter of ferrite having a number of holes after firing gets smaller compared to that of ferrite having less holes. Therefore, humidity resistance and the like tend to deteriorate.
In view of the above, for a method of combining magnetic materials with low dielectric constant non-magnetic materials, there needs to be improvements for the following five problems. Specifically, decrease of sinterability, decrease of permeability μ, lower frequency of frequency property of permeability μ, smaller effects of decreasing of dielectric constant, and deterioration of insulation resistance. It is considered difficult to provide a multilayer coil having high impedance in GHz band and also simultaneously to resolve the above problems.    Patent Document 1: Japanese Published Unexamined Application No. H11-026241    Patent Document 2: Japanese Published Unexamined Application No. 2002-175916    Patent Document 3: Japanese Published Unexamined Application No. 2004-297020